Программно конвертировать Word (docx) в PDF

Другой вариант, даже если он требует некоторой доработки: установите OpenOffice на сервер и, используя библиотеки UNO (включая их в виде сборок в вашем приложении), вы сможете открыть документ docx и сохранить его в PDF напрямую.
Через несколько минут выкладываю пример...

ЧАСТИЧНЫЙ ПРИМЕР:
Это класс, который я создал давным-давно и использовал для преобразования файлов в pdf.

using unoidl.com.sun.star.lang;
using unoidl.com.sun.star.uno;
using unoidl.com.sun.star.container;
using unoidl.com.sun.star.frame;
using unoidl.com.sun.star.beans;
using unoidl.com.sun.star.view;
using System.Collections.Generic;
using System.IO;

namespace QOpenOffice
{
    public enum AppType
    {
        Writer,
        Calc,
        Impress,
        Draw,
        Math
    }

    public enum ExportFilter{
        Word97,
        WriterPDF,
        CalcPDF,
        DrawPDF,
        ImpressPDF,
        MathPDF
    }

    class OpenOffice
    {
        private XComponentContext context;
        private XMultiServiceFactory service;
        private XComponentLoader component;
        private XComponent doc;

        private List<string> filters = new List<string>();

        #region Constructors
        public OpenOffice()
        {
            /// This will start a new instance of OpenOffice.org if it is not running, 
            /// or it will obtain an existing instance if it is already open.
            context = uno.util.Bootstrap.bootstrap();

            /// The next step is to create a new OpenOffice.org service manager
            service = (XMultiServiceFactory)context.getServiceManager();

            /// Create a new Desktop instance using our service manager
            component = (XComponentLoader)service.createInstance("com.sun.star.frame.Desktop");

            // Getting filters
            XNameContainer filters = (XNameContainer)service.createInstance("com.sun.star.document.FilterFactory");
            foreach (string filter in filters.getElementNames())
                this.filters.Add(filter);
        }

        ~OpenOffice()
        {
            if (doc != null)
                doc.dispose();
            doc = null;
        }
        #endregion

        #region Private methods
        private string FilterToString(ExportFilter filter)
        {
            switch (filter)
            {
                case ExportFilter.Word97: return "MS Word 97";
                case ExportFilter.WriterPDF: return "writer_pdf_Export";
                case ExportFilter.CalcPDF: return "calc_pdf_Export";
                case ExportFilter.DrawPDF: return "draw_pdf_Export";
                case ExportFilter.ImpressPDF: return "impress_pdf_Export";
                case ExportFilter.MathPDF: return "math_pdf_Export";
            }
            return "";
        }
        #endregion

        #region Public methods
        public bool Load(string filename, bool hidden)
        {
            return Load(filename, hidden, "", "");
        }
        public bool Load(string filename, bool hidden, int filter_index, string filter_options)
        {
            return Load(filename, hidden, filters[filter_index], filter_options);
        }
        public bool Load(string filename, bool hidden, string filter_name, string filter_options)
        {
            List<PropertyValue> pv = new List<PropertyValue>();
            pv.Add(new PropertyValue("Hidden", 0, new uno.Any(hidden), PropertyState.DIRECT_VALUE));
            if (filter_name != "")
            {
                pv.Add(new PropertyValue("FilterName", 0, new uno.Any(filter_name), PropertyState.DIRECT_VALUE));
                pv.Add(new PropertyValue("FilterOptions", 0, new uno.Any(filter_options), PropertyState.DIRECT_VALUE));
            }

            try
            {
                doc = component.loadComponentFromURL(
                    "file:///" + filename.Replace('\\', '/'), "_blank",
                    0, pv.ToArray());
                return true;
            }
            catch
            {
                doc = null;
                return false;
            }
        }
        public bool Print()
        {
            return Print(1, "");
        }
        public bool Print(int copies, string pages)
        {
            List<PropertyValue> pv = new List<PropertyValue>();
            pv.Add(new PropertyValue("CopyCount", 0, new uno.Any(copies), PropertyState.DIRECT_VALUE));
            if (pages != "")
                pv.Add(new PropertyValue("Pages", 0, new uno.Any(pages), PropertyState.DIRECT_VALUE));
            //if (doc is XPrintable)
            try
            {
                ((XPrintable)doc).print(pv.ToArray());
                return true;
            }
            catch { return false; }
        }
        public bool Save(string filename, ExportFilter filter)
        {
            return Save(filename, FilterToString(filter));
        }
        public bool Save(string filename, string filter)
        {
            List<PropertyValue> pv = new List<PropertyValue>();
            pv.Add(new PropertyValue("FilterName", 0, new uno.Any(filter), PropertyState.DIRECT_VALUE));
            pv.Add(new PropertyValue("Overwrite", 0, new uno.Any(true), PropertyState.DIRECT_VALUE));
            try
            {
                filename = filename.Replace("\\", "/");
                ((XStorable)doc).storeToURL("file:///" + filename, pv.ToArray());
                return true;
            }
            catch { return false; }
        }
        public bool ExportToPdf(string filename)
        {
            filename = Path.ChangeExtension(filename, ".pdf");
            bool ret = Save(filename, "writer_pdf_Export");
            if (!ret) ret = Save(filename, "impress_pdf_Export");
            if (!ret) ret = Save(filename, "calc_pdf_Export");
            if (!ret) ret = Save(filename, "draw_pdf_Export");
            if (!ret) ret = Save(filename, "impress_pdf_Export");
            if (!ret) ret = Save(filename, "math_pdf_Export");
            return ret;
        }
        public void Close()
        {
            doc.dispose();
            doc = null;
        }

        public bool New(AppType app, bool hidden)
        {
            try
            {
                string sapp = "private:factory/";
                switch (app)
                {
                    case AppType.Writer:
                        sapp += "swriter";
                        break;
                    case AppType.Calc:
                        sapp += "scalc";
                        break;
                    case AppType.Impress:
                        sapp += "simpress";
                        break;
                    case AppType.Draw:
                        sapp += "sdraw";
                        break;
                    case AppType.Math:
                        sapp += "smath";
                        break;
                }
                PropertyValue pv = new PropertyValue("Hidden", 0, new uno.Any(hidden), PropertyState.DIRECT_VALUE);
                doc = component.loadComponentFromURL(sapp, "_blank", 0, new PropertyValue[1] { pv });
                return true;
            }
            catch
            {
                doc = null;
                return false;
            }
        }
        #endregion


        #region Properties
        public List<string> Filters
        {
            get { return filters; }
        }
        #endregion
    }
}